This invention relates to a method and apparatus for locating the position of a stylus, or pen, on a grid of electrical conductors. This technique is also applicable to other devices such as a mouse or puck. These grids are generally employed in digital tablets for use in inputting data to a digital computer system.
FIG. 1 illustrates a prior art apparatus generally indicated as 100 for determining the location of a stylus in the vertical, or Y, direction. A typical grid determines the location of the stylus in the horizontal, or X, direction as well; this being omitted from the drawing for the sake of clarity. The grid comprises 16 horizontal conductive lines 102a through 102p, each of which is connected to a respective driver 104a through 104p. The conductors of the grid are substantially parallel to each other and are equally spaced one from the other. The conductors are normally formed as printed circuit conductors on one side of a printed circuit board. The surface of the grid may be covered with a protective layer of plastic or other non-conductive material.
In operation, a pen or stylus containing a capacitive or inductive pick up is placed on the surface of the grid. The drivers are sequentially energized by suitable electronics (not shown) connected to each of the input terminals 106a through 106p of the drivers. The drivers in turn energize the conductors 102 to which they are connected. Each time a conductor is activated, the voltage induced in or capacitively coupled to an inductive or capacitive pickup in the stylus is amplified and the amplitude of this signal is measured and stored. The largest stored amplitude represents the conductor of the grid to which the stylus is closest, thus giving a coarse determination of the location of the stylus. The exact position of the stylus between adjacent conductors may be located with greater precision by applying well-known interpolation techniques to the stored voltages.
The main disadvantage of this prior art technique is the requirement that a driver be provided for each one of the conductors in the grid. In grids employing a large number of conductors, a correspondingly large number of drivers is required. The large number of drivers and large number of input lines that are required may dictate that the drivers be placed on a separate printed circuit board from the grid. This in turn requires a large number of interconnects between this separate board and the grid; that is, one connection per conductor of the grid. In view of the fact that it is common to have a horizontal (X) grid on one side of the printed circuit board and a vertical (Y) grid on the other side of the same printed circuit board, a large number of connections must be made to such a grid. This increases the size and cost while at the same time reducing the reliability and serviceability of the system. In addition, the determination of the position of the stylus may require that all of the conductors in the grid be activated, depending on the location of the stylus. For example, if the stylus is located near one of the last conductors to be activated, then the search to find the position of the stylus will require the amount of time necessary to sequentially activate all of the conductors. In a very large grid, there may not be an adequate amount of time to perform this search and have an adequate number, of data points to define the path taken by a moving stylus.
Another prior art technique, which is utilized in the Model 4680 tablet manufactured by assignee's Applicon division, is to divide the grid into four blocks, each of which contains a plurality of juxtaposed conductors. The position of the stylus is determined by simultaneously activating one conductor from each block, each of which has the same relative position within the block. Once a stylus signal is detected, each of the conductors is separately energized to determine the block in which the stylus is located. Data points about this conductor are subsequently obtained to determine the exact position of the stylus by well known interpolation techniques. This technique solves the problem of the time required to scan all of the conductors of a large grid, here typically 440 conductors in the horizontal direction. However, it still requires the use of one driver per conductor of the grid, with the attendant problems discussed above.
Other prior art techniques include a grid network having a plurality of conductors defining a Gray Code pattern as shown in U.S. Pat. No. 3,709,956. Each of the conductors of the grid corresponds to a bit in the Gray Code. The stylus is located by sequentially passing a current through each of the conductors and detecting the phase of the voltage induced in an inductive pickup located in the stylus. Another Gray Code technique is shown in U.S. Pat. No. 3,819,857. Gray Code techniques do not lend themselves to interpolation between the conductors to obtain a more accurate location of the stylus.
U.S. Pat. No. 3,342,935 shows a system which utilizes a pulse technique to obtain a coarse location and a phase measurement to obtain a fine location of a stylus. This requires a counter to develop the pulses utilized for the coarse position location and a sine wave generator, phase shifters and coupling diodes for the fine position location.